A known ultrasonic probe used in an ultrasonic diagnostic apparatus performs ultrasonic scanning by mechanically oscillating or rotating an ultrasonic transducing part for transmitting and receiving ultrasonic waves in an outer case filled with an acoustic medium. In such an ultrasonic probe, a material having an acoustic impedance similar to that of a living body, specifically a liquid such as liquid paraffin, is used as the acoustic medium (for example, Patent Documents 1 to 3).    Patent Document 1: JP 2001-178727 A    Patent Document 2: JP 3(1991)-32652 A    Patent Document 3: JP 4(1992)-84946 A